Gratuitous ARP is wrong

[hannesm]

RFC 5227, section 3 (https://tools.ietf.org/html/rfc5227#section-3) says it should be a ARP request with spa = tpa (mirage-tcpip uses a ARP reply with tpa = broadcast). Section 3 also makes it clear why an ARP reply is a bad idea. Related to mato/docker-unikernel-runner#1 (setting arp_accept (somewhere in proc on linux) to the bridge and sending an ARP request solves the issue (just verified with @mato)).

Discovered while reimplementing ARP (https://github.com/hannesm/arp - https://hannes.nqsb.io/Posts/ARP)

[yomimono]

Thanks for the report - we're indeed doing it pretty wrong. I'll fix this up.

[hannesm]

I'd guess a branch off 2.8.0 release with having only this fixed (and a 2.8.1 release) would make people, instead of waiting till master is in a state to be released (which requires new mirage-types release)....